Lifting ship wrecks in swell conditions

Abstract

When a shipwreck lies in an undesirable place it can be lifted out of the water or moved to larger water depths(scuttling). Commonly used methods for this procedure are the use of floating sheerlegs, crane ships or chain pullers. Floating sheerlegs and crane ships can be used for lifting the shipwreck through the water column, where chain pullers are mostly used to move the shipwreck to larger water depths. The scuttling procedure will be the main focus of this thesis. Due to the enclosed water in the shipwreck, the dynamic mass is relatively large compared to the submerged weight so the dynamic behaviour will play an important role during the lift procedure in waves. Especially swell conditions can cause large heave motions which causes high peak loads at the shipwreck and the lift system. Therefore a new concept needs to be designed to reduce the peak loads and increase the workability. With the help of a concept analysis the chain pullers on a barge are chosen as the lift unit. For this lift unit five different groups of load reduction methods are defined namely Active Heave Compensation (AHC), Passive Heave Compensation (PHC), Hawsers, Fenders and additional buoyancy. For these load reduction methods six different systems are defined which has led to a broad scale of possible concepts. The hydrodynamic properties of the barge and the shipwreck are determined with Ansys AQWA. To get a first insight in the dynamic behaviour and the performance of the load reduction methods a Matlab model is made. Here the barge is modelled as a 3 Degrees Of Freedom (DOF) system (heave, roll and pitch) and the shipwreck as a 1DOF system (heave). From the results can be seen that the PHC can fulfill the load reduction criteria sufficiently and will be chosen as load reduction method. With the help of a Multi-Criteria Analysis (MCA) the different systems are ranked based on ten different criteria and one system is chosen as concept. For the chosen concept an OrcaFlex model is made to model its dynamic behaviour in more detail. The barge and the shipwreck are now each modelled as a 6DOF system and the mooring lines are also added to the system. Beside that the response to irregular waves can be calculated with frequency dependent added mass and damping values of Ansys AQWA. The PHC is now modelled as a spring dashpot system based on experimental data from companies. Beside that the linear soil model is extended with a friction force and also a non-linear soil model can be implemented to take into account suction forces The natural frequencies of the coupled system (barge+shipwreck) without the PHC are in the range of the wave frequencies for the anti-symmetric mode of surge and sway and for the symmetric mode of heave, roll and pitch. The response in regular and irregular waves shows that there is a significant reduction of the maximum tension after implementing the PHC system. After performing a workability study can be seen that at Hs=2m and T=8.5 sec the peak tensions are still below the maximum peak tension for the base case scenario. From the sensitivity analysis can be concluded that a small change in added mass and submerged weight of the shipwreck can have a significant influence on the chain tensions. Frequency dependent added mass and damping of the shipwreck gives different dynamic behaviour and also can have an influence on the chain tensions and workability. For the linear soil model the soil stiffness barely has an influence on the chain tensions. Suction forces can have an influence on the chain tension dependent the type of soil and the penetration depth.